In many applications, a transmission line is used to transport a high-frequency signal from one location to another. In an example, a transmission line on a printed circuit board is used to transport a signal from one integrated circuit to another. In another example, a transmission line on an integrated circuit is used to transport a signal from one location to another within the integrated circuit.
FIGS. 1A and 1B are block diagrams showing examples of a typical conventional data transmission system 10. Data transmission system 10 is composed of a transmitter 12 and a receiver 14 electrically coupled by a transmission line 20. Transmission line 20 is depicted as a coaxial transmission line for the purpose of illustration only. Any other type of transmission line may be used as transmission line 20. Transmission line 20 has an input 22 and an output 24. Transmitter 12 is electrically coupled to the input 22 of transmission line 20. The output 24 of transmission line 20 is electrically coupled to the input 16 of receiver 14. In the example shown in FIG. 1A, the output 24 of transmission line 20 is DC coupled to the input 16 of receiver 14. In the example shown in FIG. 1B, the output 24 of transmission line 20 is ac coupled to the input 16 of receiver 14 by a capacitor 32 and input 16 is biased by a resistor 34 connected to a voltage source 36. Receiver 14 has a threshold level and interprets a level at input 16 greater than the threshold level as a logical 1, for example, and a level less than the threshold level as a logical 0, for example, or vice versa.
In conventional data transmission system 10, transmitter 12 imposes a DC-balanced signal of the input 22 of transmission line 20 for transmission to receiver 14. Transmitter 12 generates such DC-balanced signal by mixing the original data to be transmitted with a pseudo-random bitstream. The process is reversed in receiver 14 to recover the original data. The DC-balanced signal is DC-balanced in the sense that the temporal density of high states in the signal is equal to the temporal density of low states in the signal. As a result, the DC-balanced signal has a substantially constant average DC level.
A conventional transmission line-based data transmission system such as data transmission system 10 suffers from a loss of data at the start of each transmission. Such data loss is undesirable.
Some conventional data transmission systems additionally have an auxiliary receiver (not shown) collocated with the transmitter and an auxiliary transmitter collocated with the receiver. An error detection process applied to the data output by the receiver indicates that data has been lost at the beginning of the transmission. The auxiliary transmitter then sends a message to the auxiliary receiver to cause the auxiliary receiver to instruct the transmitter to re-send the data. This retransmission process degrades the transmission efficiency of the data transmission system. Moreover, the need for an auxiliary transmitter and an auxiliary receiver increases the cost of transmission system and increases its power consumption.
Accordingly, what is needed is a way to transmit a DC-balanced signal via a transmission line in which data is not lost at the start of each transmission.